MOSFET (Metal-Oxide-Silicon Field Effect Transistors) devices comprise a gate electrode, a gate dielectric, and a channel formed in or on a substrate. The gate dielectric dielectrically couples the channel to the gate electrode so that the gate electrode potential can modulate the conductivity of the channel. There is a trend to replace semiconductor gate electrodes with metal gate electrodes. Metal gate electrodes offer the advantages of reducing the sheet resistance, eliminating the semiconductor gate depletion effect, and controlling the work function independently from the doping of the junction regions.
Metal gate electrodes can be formed by full silicidation (FUSI) of the semiconductor gate electrode with a metal. The semiconductor gate electrode may be a polysilicon gate electrode. The metal may be a refractory metal such as tungsten (W), noble metals such as platinum (Pt), near noble metals such as nickel Ni, transition metals such as titanium (Ti), or any combination thereof. During this silicidation process, the gate electrode is converted into a silicide.
Various manufacturing methods and materials exist to form FUSI gate MOSFET devices using full silicidation of the semiconductor gate electrode. W. Maszara et al. discloses in “Transistors with Dual Work Function Metal Gates by Single Full Silicidation (FUSI) of Polysilicon gates,” IEDM proceedings 2002 p367-370, the use of nickel-silicides to form a FUSI gate. According to this approach, the FUSI gate of a MOSFET device is formed during a single silicidation step using nickel as silicidation metal. The work function of the FUSI gate can be tuned by varying the thickness of the as-deposited nickel layer that will determine which nickel-silicide phase will be formed during silicidation.
U.S. Pat. No. 6,905,922, hereby incorporated by reference in its entirety, discloses a method for manufacturing FUSI gate MOSFET devices. According to this method, the MOSFET device, comprising a semiconductor gate electrode, source/drain regions and spacers alongside the gate electrode, is covered with a dielectric layer. By removing this dielectric layer from the semiconductor gate electrode, metal can be deposited on top of the semiconductor material to allow forming of the silicide.
As known from M. A. Nicolet et al in “VLSI electronics: Microstructure Science, Vol. 6,” editors: N. G. Einspruch and G. B. Larrabee, Academic Press, New York, 1983, chapter 6, pages 455 to 459, when forming a silicide by the reaction between a semiconductor material and a metal, the volume occupied the silicide may be larger than the volume occupied by the semiconductor material, where the volume expansion is essentially a one-dimensional expansion and the thickness of the silicide layer is larger than the thickness of the as-deposited semiconductor layer.